Seismometer



Sept. 22, 1953 Filed Oct. 5, 1949 5 Sheets-Sheet l F/Gl.

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ATTORNEYS R- G PIETY SEISMOMETER Sept' 22 1953 3 sheets-sheet Filed Oct. 3, 1949 mnnl- FIG. 3,

To AMPLIFIER T R 0 P P U s 2 7G \N T L U s N INVENTOR. l R.G. PIE-ry, BM gru I To AMPLIHER ATTORNEYS Sept. 22, 1953 R. G. PIETY 2,653,306

sEIsMoMETER Filed oct. s, 1949 s sheets-sheet s NVEN RG. PIE BYMQV A TTORNEYS Patented Sept. n, 1953 UNITED STATI-:s wi'riazNT oFF'Ic'E SEISMOMETEB n Raymond, o. riezy, Barium ons., mmm

Phillipe Qompany. a corporation to of.

Petroleum appunti@ omite a, me, sam No. uam

(ci. :4a-rn l2 Claims. *l l l Ihis invention relates to selsmometers and to a method for-making seismometers. In another aspect. it relates to seismometers in which 'seislstrips or plates arranged as a condenser, and this relative movement would change the capacity between the condenser plates. However, diillculties have arisen in the practical application of this principle to the actual construction of seisthefollowing detailed description, taken in con- Junction with the accompanying drawings. in which:

Figure l is a vertical sectional view of the seismometer of this invention;

Figure 2 is a vertical sectional view of a modified form of seismometer;

Figure 3 is a schematic circuit diagram illustrating the electrical connection of the seismometer to an amplifier unit;

Figure 4 is a vertical sectional view, partially in elevation. of a modified form of seismometer;

mometers, in that a large surface area is required to produce measurable capacitance changes in response to the small movements produced between the plates by seismic waves. Further, a

.very close and uniform spacing must be maintained between the plates or the capacitance thereof will not vary linearly with the amplitude or the seismic waves.

In accordance with the present invention, difficulties heretofore encountered in the construction of capacitance type seismometers are overcome by utilizing screw threads as the elements between which the capacitance to be measured is developed. Screw threads may be accurately machined and held to close tolerances so that it is possible to obtain a large surface area between the conductors deilning the condenser while maintaining the requisite close spacing. The two elements deilning the condenser are mounted by a novel spring suspension so that seismic waves incident upon the seismometer produce relatively large capacitance variations representative of the seismic waves.

It is an object of the invention to provide an improved seismometer of the capacitance type.

It is a further object of the invention. to provide an improved method for making a seismometer of the capacitance type.

lt is a still further object provide a seismometer of extremely small s e and rugged construction having an output comparable with that of much large seismometers of the moving coil type.

Various other objects, advantages and features of the invention will become apparent from and Figure 5 is a schematic circuit diagram illustrating the electrical connection of the seismometer of Figure 4 to an amplifier unit.

Referring now tothe drawings in detail, and

particularly to Figure l, the seismometer comprises a cylindrical casing I0 having a central passage formed therein. The upper part II of the casing is machined to form helical threads I2 and the lower lpart I3 of the casing is mafchined to form helical threads- I4 of opposite pitch. In the present embodiment of the invention, the casing is formed from conductive metal so that the threads I2 define a continuous helical metal surface I5 and .the threads Il define a continuous helical metal surface I6. The upper section Il and the lower section I3 of the casing lare separated and electrically insulated from each other by a flanged portion IB of a split insulating sleeve I9, this sleeve deilning an inner cylindrical passage 2li. It will be noted that the threads I2. I4 adjacent sleeve I9 have their inner surfaces machined away and that the out away threads, such as threads 2l and 22, have their inner surfaces tapped to engage threads 23 formed on the outer surface of insulating member I9. In this fashion, the insulating member I9 supports the adjacent sections of the casing and insulates them from each other.

In accordance with-the invention, a metal armature 25 is freely suspended for longitudinal movement within the passage in casing III by spring suspension units 21 and 2B. 'I'he armature includes an upper threaded section 29, complementary with threaded portion I2 and a lower threaded section 30 of opposite pitch which is complementary with the threaded portion Il. 'I'he threads at the central region of the armature are cut away to form a tubular section disposed within the sleeve I9. The armature threads are of slightly less width than the threads I2. I4 sothat the upper threads 29 define a aesasoe deiinedbythethreads lLmdthethreadsildefine s continuous helical surface Il of conductive metal which is closely spaced to the continuous conductive surface I6 defined by the threads Il in the casing.

Assumingthatthecasingismoveddownwardly relative to the armature in response to seismic incident upon the seismometer, the spacing between surfaces Il, ll is decreased and the .spacingbetweensurfacesltisincreased Asa result, the capacitance between the upper section of the armature and the casing is increased while the `capacitance between the lower section of the armature and the casing is decreased. the changes in capacitance being proportional to the amplitude of the seismic waves incident upon the seismometer. When the casing moves upwardly relative to the amature, an opposite action occurs with the result that the capacitance between the amature and the lower section of the casing is increased while the capacitance between the amature and the upper section of the casing is decreased. It will be understood that the amature remains in a substantially stationary position due to its inertia when the casing is removed relative to the amatureby seismic waves incident upon the seismometer.

The spring suspensions 21, 2l provide a free suspension of the armature relative to the casingl thereby allowing the described movement between -these parts responsive to seismic waves to take place. Spring suspension 2l includes a nat spiral spring l5 having its inner end secured between an insulating stud il and a washer I1, these parts being secured to the lower end of amature Il by a screw I8 and a metal washer I9. The outer end of spiral spring Il is mounted between a ange 4l formed on the casing and a spacer 4I which is forced into engagement with the spring by a lock nut I2 threaded within the casing. `A cap Il is provided at the lower end 'of the casing to seal the seismometer unit. Spring suspension 21 is of similar construction and includes a flat spiral spring Il, a stud I6, an insulating washer l1, a screw-38, a spacer 4|, and a lock nut Il whose structure and function are similar to those of the corresponding parts of spring suspension 28. Metal washer 3l, however,

of accurately regulating the original vertical position of the armature with respect to the casing. That is, adjustment of set screw 55 changes the initial spacing between conductive surfaces Il. ll and between conductive surfaces i6 and Il. The flat spiral springs 35 tend to maintain the armature in a predetermined position such that the distance between surfaces I5. 3| is equal to the distance between surfaces IB and 33. Accordingly, springs 35 provide the primary force for holding the armature in desired position relative to the casing while the piston 48 and its assocmted springs provides a means for making minor adjustments of the position of the a1'- mature in the casing.

A modined form of seismometer is shown in Figure 2 which shows only the central portion of the armature and casing. It will be understood that both ends of the structure are extended and provided with spring suspension units in the manner illustrated by Figure l. In this modification, a casing 58 is provided with interior threads I9 of square or rectangular cross section, the casing and threads being formed from a suitable insulating material, such as Bakelite. An armature 60 is suspended within the casing 5l and this amature is provided with threads 6I of rectangular cross section which are complementary with the threads 59 formed in the casing. The armature is formedy from conductive metal and the threads therein are of lesser width than the space between the threads 59 formed in the casing. Accordingly. the spring suspension units, not shown, allow relative movement between the armature and casing in the manner described in connection with Figure l.

In forming the stator, the threads 59 have a metal plating applied thereto of a conductive is replaced by a pair of spring retainer plates Il, 4l which support one end of a helical spring Il. The other end of spring It is secured to a pfston assembly 4l by a .spring retainer plate 4I threaded on a metal stud lili. The piston as sembly is freely suspended for longitudinal movement within the casing lil and it is urged in an upward direction by a helical spring Il which is mounted between piston assembly It and a nut 53. The upper end of the casing is closed by a cap Il which carries a set screw Il bearing on stud 5l and has an opening 58 therein to carry the electrical conductors, not shown, attached to the movable parts of the seismometer.

In the operation of the spring suspension unit, spring Il urges piston assembly lli into engagement with set screw 65. The spring It, which is maintained in tension, exerts an upward force upon the armature 25 which is proportional to the distance between piston lt and the armature. Accordingly, by moving set screw 55, the vertical position of piston ll may be varied with resultant variation in the upward tension placed upon the armature by spring This structure thus provides a simple and emcient method material, such as copper. During the plating operation, the conductive metal is applied to the entire surface of the threads. After the plating operation is completed, the inner or peripheral edges of the threads 59 are machined, thereby to remove the plating together with a. smal1 portion of the thread material itself. As a result, the plate material remaining at the sides of the thread defines two continuous helical plates 63 and 64, one at each side of the threaded portion 29. These plated strips cooperate with the facing surfaces of the armature threads to form condenser elements, the capacitance of which is changed responsive to relative movement between the armature and casing. Assuming that the armature is originally positioned so that the threads 8i lie midway between the threads 59, upward movement of the casing relative to the amature causes the surfaces B5 of the armature to move closer to plated strip 6I and surfaces 6B of the amature to move away from plated surfaces 6l. As a result, the capacitance between the armature and plated strip B3 is increased while the capacitance between the armature and plated surface El is decreased. Downward movement of the casing relative to the armature produces an opposite result, that is, the capacitance between the armature and plated strip 6I is increased while the capacitance between the armature and plated strip 63 is decreased. These changes in capacitance are proportional to the degree of relative movement between the casing and armature, and hence the amplitude of the seismic waves incident upon the seismometer.

It will be apparent that I have provided a seismometer of the variable capacitance type B in whiohrelaiivelymovable surfaces are closely spaoedthroughoutalargeareatoprovideahigh capacitaneeandahishchangeincapacitencein responsetorelativelrmvementbetweenthecon- The accura of spacing results nent parts. The seismometer of this invention is particularly adapted for use in a radio seismic system, and it simpliiles the feedback problems of automatic volume control in that the amp1i.

tude of the power supply can be controlled. This capacitor typev of seismometer lends itself to multi-unit operation over a single pair of leads.

since each of a plurality of seismometers can be supplied with a diil'erent frequency with separation of the frequencies being made at the receiving station.

A preferred method of connectingthe seismometer of this invention in an amplifier circuit is shown by Figure 3. 'Ihis circuit includes an electron tube il having an anode, a cathode. and a control grid. The anode and cathode are connected to a subsequent ampliiier stage in the usual manner while the control grid is connected by a lead 'il to the amature 2i of the capacitive seismometer. 'Ihe two remaining stator elements of the seismometer are connected to leads ll and 12, these wires also being connected to a current source 13, and an inductance 1l. The inductance is shuntedl by` a pair of series-ccnnected condensers 1I and 16, the junction between which is grounded. A condenser l1 is connected between leads l0, 1| and a condenser 1l is connected between leads I0 and l2. When the armature is positioned midway between the stator plates, the circuit is balanced and a zero average potential is applied to the control grid of an electron tube 69. When the armature moves toward either of the stator plates responsive to seismic waves incident upon the seismometer, a voltage is impressed upon the control grid of tube I9 which is directlyfproportional to the ratio of the increase in capacity on one side of the seismometer to the decrease in capacity on the opposite side, the polarity of the voltage being controlled by the sense of amature deviation. Where current source 1l supplies direct current, the voltage applied to the control grid of tube I! is proportional to the velocity of movement of the armature relative to the casing whereas, when source 'Il supplies alternating current. the voltage applied to the control grid is proportional to the displacement of the armature relative to the casing. It will be apparent from this 0 circuitthatthsuseofthedoublesectioncapacitance doubles the output over the value which -would be obtained if only a single section were utilised. With this circuit, the ampliner output voltage faithfulb reproduces the seismic waves` incident upon the seismometer ReferringnowtoFigureLIh'ave shown a-still.

further modification of the seismometer of Figures i and 2. This unit includes a cylindrical casing Il within which a suspended element Il is movable in a longitudinal direction. The element Il is'supported by suitable spring suspensions. not shown. which bias it to a predeter-` mined position relative to casing Il but permit shifting of the suspended element responsive to c seismic waves incident upon the seismometer. Two sets Il and Il of helical or spiral grooves are cut in the interior wall of casing I I and these grooves receive respectively. a pair of helical metal condenser elements Il and Bl, each element having a tapered outer edge 88 which ilts into the corresponding grooves 82 or I3 together with a thin flat inner portion 81 which protrudes adially inward from the corresponding groove .I or Il. 'I'he elements Il and le may be conveniently formed upon a lathe and they are iltted into their respective grooves in much the same manner as the nut is screwed onto a bolt. In similar fashion, the suspended element Il is provided with two sets BB and Blof helical or spiral grooves ,into which are iltted a pair of helical metal condenser elements 90 and 9|. These elements are similar to the elements 8l and 85 except that the tapered portion 92 ofeach of the elements 90- and 9| is formed along the inner edge of the element rather than along the outer edge, the flat plate portions protruding outwardly from the suspended element 8i.

It will be noted, therefore, that the elements l0, 9| alternate with the elements 84, 85 when the complete seismometer unit is assembled and that relative movement between casing 80 and suspended element 8| causes the capacitance between the sets of condenser elements to vary in accordance with the magnitude of the disturbance incident upon the seismometer. An important advantage of this construction is that the space in which the condenser elements are mounted is very efilciently utilized, and. oi'

'merely by unscrewing them from the casing or suspended element.

In Figure 5, I have shown a circuit for transforming the capacitance variations produced by the seismometer into electrical voltages representative thereof. It will be noted that the condenser elements BI, 85. 9D and 9i are represented symbolically upon this diagram, the plates 04 and Il being connected to the respective terminals of an inductance 94 which is shunted by an alternating current generator 95 and a pair of series connected condensers 96, 91 the junction between which is grounded. The elements and Il, separated by insulating support 8|, are connected to the respective control grids of ampliiier tubes ll and ls, the cathodes of which are grounded, and the anodes ofwhich are connected in push-pull circuit arrangement with subsequent amplifier stages. The operation of the circuit is similar to that of Figure 3 but the arrangementof the condenser elements electrically is such that they are adapted for connection to a push-pull amplifier, rather than to a single ended amplifier as in Figure 3. The described seismomer toge'tner with the circuit of l Figure responds, in a very sensitive manner,

to movement of the ground upon which the seismometer is positioned resulting from `seismic thereof, it is to be understood that this description is illustrative only and is not intended to limit the invention, the scope of which is defined by the appended claims.

Having described my invention, I claim:

l. A capacitive seismometer comprising, in combination, a casing having an interior passage, a helical threaded portion formed within said passage, said threaded portion raving a continuous helical surface of conductive metal, an armature having an exterior helical threaded portion including a continuous surface of conductive metal, said threaded portion of said armature being interleaved with the threaded portion in said passage, one of said threaded portions being of smaller width than the other threaded portion, and means for freely suspending said armature within said passage whereby relative longitudinal axial movement between said armature and said casing produces a change in capacitance between said metal surfaces.-

2. A capacitive seismometer, in combination, a casing having an interior passage, a helical threaded portion formed within said passage, said threaded portion having two continuous helical surfaces of conductive metal, an armature having an exterior helical threaded portion including a conductive surface contiguous to said first metal surface and a second conductive surface contiguous to said second metal sm'face, the threaded portions of said armature and said passage being interleaved, and means for freely suspending said armature within said passage whereby relative movement between said armature and said casing moves one set of conductive surfaces toward each other and the other set of conductive surfaces away from each other.

3. vA capacitive seismometer comprising, in combination, a casing having an interior passage, a helical threaded portion formed within said passage, said threaded portion having two continuous helical surfaces of conductive metal, an armature having an exterior helical threaded portion including a conductive surface contiguous to said first metal surface and a second conductive surface contiguous to said second metal surface, the threaded portions of said armature and said passage being interleaved, a pair of fiat spiral springs mounted, respectively, at opposite ends of said passage, said armature being freely suspended by said springs whereby relative movement between said armature and said casing moves one set of conductive surfaces toward each other and the other set of conductive surfaces away from each other.

4. A capacitive seismometer comprising, in combination, a casing having an interior passage, a helical threaded portion formed within said passage. said threaded portion having two continuous helical surfaces of conductive metal, an armature having an exterior helical threaded portion including a conductive surface contiguous to said first metal surface and a second conductive surface contiguous to said second metal surface, the threaded portions of said armature and said passage being interleaved, a pair of fiat spiral springs mounted, respectively, at opposite ends of said passage, said armature being freely suspendnection with a present, preferred embodiment ed by said springs whereby relative movement between said armature and said casing moves one set of conductive surfaces toward each other and the other set of conductive surfaces away from each other, and a spring biasing said armature to a position where said conductive surfaces are equally spaced from said metal surfaces.

1. 5. A capacitive seismometer comprising, in combination, a casing having an interiolpassage, two helical threaded portions of opposite pitch formed within said passage, each threaded portion having a continuous helicalsurface of conduetivemetal, an armature having two exterior helical threaded portions of opposite pitch which are interleaved with the threaded portions in said passage, each threaded portion on the armature including a continuous surface of conductive metal closely spaced to one of the metal surfaces formed in said casing. and means for freely suspending said armature within said passage whereby relative movement between said amature and said casing produces a change in capacitance between said metal surfaces.

6. A capacitive seismometer comprising, in combination, a casing having an interior passage, two helical threaded portions of opposite pitch disposed in adjoining relation within said passage, the adjacent ends of said threaded portions being cut away to form a cylindrical bore, a sleeve of insulating material disposed within said bore and having a flanged portion dividing said casing into two sections which arev electrically insulated from each other, said threaded portions each having a continuous helical surface of conductive metal, an armature having two exterior helical threaded portions in said passage. the threaded portions of said armature and said passage being interleaved, said armature including a cut away cylindrical portion between said threaded portions which fits within said insulating sleeve, each threaded portion of the armature having a continuous helical surface of conductive metal which is 'closely spaced to a corresponding metal surface in said casing, and means for freely suspending said armature within said passage whereby relative movement between said armature and said casing produces an increase in capacitance between one set of metal surfaces and a decrease in capacitance of similar magnitude between the other set of metal surfaces.

7. A capacitive seismometer comprising, in combination, a casing having an interior passage, two helical threaded portions of opposite pitch disposed in adjoining relation within said passage, the adjacent ends of said threaded portions being l cut away to form a cylindrical bore, a sleeve of 'a continuous helical surface of conductive metal,

insulating material disposed within said bore and having a flanged portion dividing said casing into two sections which are electrically insulated from each other, said threaded portions each having an armature having two exterior helical threaded portions in said passage, the threaded portions of said armature and said passage being interleaved. said armature including a cut away cylindrical portion between said threaded portions which fits within said insulating sleeve, each threaded portion of the armature having a continuous helical surface of conductive metal which is closely spaced to a corresponding metal surface in said casing, an insulating support secured to each end of said armature, a flat coil spring mounted within said passage at each end of said casing,

each spring being secured to one of said insulating supports to freely suspend the armature within sulated from each other, said threaded portions 1 each having a continuous helical surface of conductive metal, an armature having two exterior helical threaded portions in said passage, the threaded portions oi' said amature and said passage being interleaved, said armature including a cut away cylindrical portion between said threaded portions which fits within said insulating sleeve, each threaded portion of the armature having a continuous helical lsurface of conductive metal which is closely spaced to a corresponding metal surface in said casing, aninsulating support secured to each end of said armature, a fiat coil spring mounted within said passage at each end of said casing, each spring being secured to one of said insulating supports to freely suspend the armature within the casing, a piston mounted for rectilinear movement at one end of said passage, a stop, means urging said piston into engagement with said stop, a spring interconnecting said piston and one of said insulating supports to maintain said armature in a position wherein both sets of contact surfaces are equally spaced from each other, and means for eii'ecting longitudinal movement of said stop to vary the position of said armature relative to said casing. i

9. A capacitive seismometer comprising, in combination, a casing of electrically insulating material having an interior passage, a helical threaded portion of rectangular cross section formed within said passage, said threaded portion having two continuous helical surfaces of conductive metal formed, respectively, on opposite sides of said threaded portion, an armature having an exterior threaded portion which is interleaved with the threaded portion of said casing, one of said threaded portions being of smaller width than the other threaded portion, said amature being formed from conductive metal, and means for freely suspending said armature within said easing whereby relative movement between said armature and said casing produces a change in capacitance between said armature and said metal surfaces.

l0. A capacitive seismometer comprising, in

combination, a casing of electrically insulating material having an interior passage, a helical threaded portion of rectangular cross section 10 formed within said passage, said threaded portion having two continuous helical surfaces of conductive metal formed on opposite sides of said thread, an armature having an exterior helical threaded portion which is interleaved with and of smaller width than the threaded portion in said passage, said armature forming a, variable capacitance unit with each of said plated sur# faces, and means for freely suspending said armature within said passage whereby relative movement between said armature and said casing increases the capacitance between said armature and one plated metal surface and decreases the capacitance between said armature and the other plated metal surface.

11. A capacitive seismometer comprising, in combination, a casing having an inner passage. a

second element positioned for longitudinal movement hr1-said passage, spring suspension mounting means for freely suspending said second element within said passage, a set of helical grooves in said casing, a set of helical grooves formed in said suspended element, and a thin spiral metal condenser element disposed in each of said groovessaid suspended element, all of said grooves havr ing the same pitch, and a thin spiral metal condenser element carried by each of said grooves whereby the turns of the four condenser elements are mounted in alternating relation proceeding from one end to the other end of said casing.

RAYMOND G. PIETY.

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